Prior to the present invention as shown by Cizek U.S. Pat. No. 3,383,435, assigned to the same assignee as the present invention and incorporated herein by reference, thermoplastic resin compositions were provided comprising a polyphenylene ether and a styrene resin. Other thermoplastic compositions are shown by Izawa et al., U.S. Pat. Nos. 3,929,930 and 3,929,931, incorporated herein by reference, which are polyphenylene ether having polystyrene grafted onto the backbone. Although these materials in the form of shaped articles have high impact strength, stiffness, good surface appearance, heat resistance and other desirable properties during or after molding, a major shortcoming of these materials as thermoplastics resins is their normally flammable nature. As a result, flame retardant and/or drip retardant agents are commonly incorporated into such blends of polyphenylene ethers and styrene resin prior to molding.
The flammability of normally flammable thermoplastic polymers have been reduced by using antimony-, halogen-, phosphorous- or nitrogen-containing additives commonly referred to as flame retardant agents. For example, aromatic phosphates such as triphenylphosphate or a combination of such compounds with other compounds such as halogenated aromatics have been added as flame retardant agents as shown by Haas, U.S. Pat. No. 3,639,506. Experience has shown, however, that although these phosphate compounds have been found to impart good flame resistant properties to such polyphenylene ether thermoplastic blends, physical properties of the molded blends such as the heat distortion temperature (HDT) are often adversely affected.
The present invention is based on the discovery that the addition of small amount of organothiophosphate, for example, a triarylthiophosphate to polyphenylene oxide-polystyrene resin blends or grafted copolymers as previously defined to provide up to 5% by weight of phosphorous, has been found to significantly reduce the flammability of the resulting molded thermoplastic materials. In addition, while imparting improved flame retardant properties to polyphenylene oxide-polystyrene blends, the organothiophosphates used in the present invention have been found to have significantly less affect on the HDT of such blends.